A recent increase of environmental awareness has been stimulating a replacement of a power-consuming illumination light source such as an incandescent light bulb with a power-saving light source. For example, as disclosed in Patent Literature 1, LEDs are coming into use in many cases instead of incandescent light bulbs. An LED has high luminous efficiency. Moreover, unlike fluorescent lamps, it is mercury-free. Therefore, the LED is highly expected as an environment-friendly light source. The LED is a point light source and has high directivity. As such, it has a feature of emitting intense light forward, i.e., to an emission direction.
On the other hand, as illustrated in FIG. 11, an incandescent light bulb 101 includes a bulb 102, a cap 103 provided at an end of the bulb 102, and a filament 104 provided inside the bulb 102. In such an incandescent light bulb 101, the filament 104 which serves as a point light source emits light. As a result, except a part that is hidden by the cap 103, the light is emitted over an almost entire circumference, or 360 degrees, as illustrated in FIG. 12.
Thus, the LED light bulb has a smaller light distribution angular range than an incandescent light bulb. Therefore, to be improved in practicality, the LED light bulb should be elaborated, in light distribution, to be more equivalent to the incandescent light bulb. Patent Literature 1, for example, discloses providing a plurality of LEDs on an outer wall of a tubular member that extends perpendicularly from a flat surface. With this configuration, it is possible to expand the light distribution angular range. However, this light bulb has disadvantages as follows: (i) The LEDs are externally visible, thereby making the light bulb less attractive aesthetically. (ii) A complex configuration of a substrate increases a cost. A technique which has no such disadvantages and can solve the foregoing problems with a more simple configuration is exemplified by the following.
In a first example, a LED light bulb is configured such that a cover is made of a highly diffusive resin or glass with a haze value of almost 99%. This makes it possible to expand the light distribution angular range.
In a second example, a LED light bulb is configured such that small-sized LED light sources are disposed to emit light in lateral directions, and that a dome-like lens (domed lens) is provided in the LED light bulb (see Patent Literature 2, for instance). In this example, light, being laterally dispersed to some extent, is diffused by a cover made of a highly diffusive resin or glass. The LED with the domed lens disclosed in Patent Literature 2 is presumably a lamp type LED. However, in terms of heat dissipation and the like, this kind of LED cannot be realized by a high-power LED. Moreover, the light bulb of Patent Literature 2 uses low-power LEDs. This requires to array a number of LEDs including LEDs surrounding the LED with the domed lens so as to emit light also in lateral directions. However, this results in a decrease in conversion efficiency from electric energy to light in a case where a number of high-power LEDs are used. In terms of efficiency, it is preferable that the light be emitted by one LED module.